The present invention relates broadly to a temperature control apparatus and in particular to a programmable digital temperature controller apparatus.
Automatic temperature control requires a feedback control system in which the controlled variable is the magnitude of the temperature. In many chemical, petro-chemical, metallurgical, and physical processes and reactions, temperature is very critical and is carefully controlled. Temperature controllers are by far the largest single group of control devices. The temperature controller is usually set to maintain a constant temperature in the system, but there are also systems in which the temperature is made to follow some prescribed program.
In automatic temperature control systems the controlled temperature is measured by means of a temperature-measuring instrument whose output or reading is compared to a desired or reference setting. If a difference exists between the measured and desired temperature, a correction is applied to an actuator which increases or decreases the amount of heat supplied to the process. The actuator controls the flow of a heated fluid such as water, oil, a molten metal, steam, vapor, or heated gases into a heat exchanger.
An important component in temperature control systems is the element which measures the temperature at the place in the process at which control is to be applied. This device, known as a temperature transducer, converts the temperature into some other quantity, such as a mechanical movement pressure, or electric voltage. This signal can be processed in a controller and applied to the actuator which controls the heat to the system.
Temperature transducers that produce mechanical movement are based on the difference in thermal expansion of two dissimilar metals. These are called lametallic thermometers. Pressure-type transducers employ a closed fluid system in which a bulb is filled with a liquid or gas. A capillary tube transmits the pressure in a conventional distant point, and a pressure expansive element, such as bellows or diaphragm, converts the pressure into mechanical movement. Electrical transducers are either of the resistance or thermocouple type. In the former, the change of resistance of a metal strip is measured in a bridge circuit. A deviation from the reference value produces a proportional signal voltage. The thermocouple type consists of two wires of different metals brazed or welded together at a junction. By placing two such junctions in series, one at a hot point and the other at a cold point, a voltage is developed which is proportional to the temperature difference between the two points.
It is of primary importance in many heating applications such as home heating or cooking that the temperature when the heat be applied in a controlled manner and that a pre-selected temperature be maintained. Once a preselected maximum temperature has been reached it is equally important, as determined by the characteristics of the heat generating appliance, that the temperature be maintained within definite predetermined limits. Large fluctuations in temperature from the preselected maximum temperature impairs the efficiency of the heating application as well as shorten the life expectancy of the heating appliance. Furthermore, uncontrolled applications of heat that vary over large time and temperature cycles result in a considerable increase in power consumption in order to maintain a given average temperature. The present invention overcomes these prior art shortcomings.